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Review and Measurements of Dynaco ST 70

sergeauckland

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Well, there's also a weird geek enjoyment to extracting high performance out of an obsolete technology (which is one major reason I design and build tube equipment).
Yes indeed, as I do when fettling my turntables, or playing around with FM transmitters is these days of digital radio.

I have a weird geek yen to build some Mullard 5-10s as they were my first 'proper' hifi amps, but I have no use for them, so probably won't get round to doing it.

One benefit of tube amplifiers is when the Bombs drop, you'll still, be able to listen to your turntable whilst all other SS phono amps will have been destroyed by the EM pulse. :eek:

S
 

DonH56

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So you'll be able to hear Nero fiddling? :)
 

rebbiputzmaker

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Well, there's also a weird geek enjoyment to extracting high performance out of an obsolete technology (which is one major reason I design and build tube equipment).
Yes life is also about enjoying oneself and having fun.
 

anmpr1

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And for some people that money is really nothing.
No question about it. But I wonder if the guy with a seven figure checking account is Wavelength's market? My guess (and I'm just guessing) is that folks with that kind of expendable dollars will simply walk into Lyric and pick out whatever Lenny is pushing that day. Customer will drop a hundred or more on electroncis and speakers, gear likely sourced from an established manufacturer. Not gear mail ordered from some guy making one offs in his basement, or barn, in addition to custom electric guitar amplifiers.

For the well heeled Lyric crowd it'll be a lot of home theater; also an expensive turntable wrapped in lucite and chrome.., mostly because it looks cool. Sure, a lot of money will change hands, but in return joe customer will have a room full of McIntosh or Levinson amps that do represent engineering value, and will probably last a very long time. And if it ever breaks, Lyric will offer a replacement while it's in the shop. Aesthetically, the fit and finish of this sort of gear makes the Wavelength thing look cheap and slummy. It's a different class, altogether.

On the other hand, it would not surprise me if 7 thousand dollar 8 watt SET guy drives a 6 year old Camry. I'm not talking about the hobbyist who builds it himself. That's a different thing altogether. I'm talking about the silver wire save the rainforest type of audiophile..., who reads about this gear in Stereophile, and believes what he is told. I'm talking about tweako true believer types. Because these amps are tweako. By definition. But although they are expensive, they are not so expensive that they can't be bought by average cats, as long as they are willing to do without other things. I've known audiophiles like this. Guys who bought weird and expensive stuff out of a strange compulsion. And all of them were making middle class dollars. Possibly it was bought on credit, or using money that would otherwise have gone toward taking waifu and kids on a Disney cruise. My guess (and again, it's just a guess) is that the super tweaky stuff is owned by more or less folks of average means, who are...well... who are audio neurotics.
 

invaderzim

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My guess (and again, it's just a guess) is that the super tweaky stuff is owned by more or less folks of average means, who are...well... who are audio neurotics.
Sadly you are probably correct in who the market is.

Audio Neurotic is a great term and I can completely identify. There was a point where whenever I'd listen to anything I'd be wondering if it would sound better with a different coupling capacitor, maybe different wires, an upgraded Pot. I wanted my system to sound the way the systems did that belonged to the people that used wonderful words and glowing phrases to describe the sound. I was always one component, one tweak away from that perfection.
 

anmpr1

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Sadly you are probably correct in who the market is.

Audio Neurotic is a great term and I can completely identify. There was a point where whenever I'd listen to anything I'd be wondering if it would sound better with a different coupling capacitor, maybe different wires, an upgraded Pot. I wanted my system to sound the way the systems did that belonged to the people that used wonderful words and glowing phrases to describe the sound. I was always one component, one tweak away from that perfection.
Yeah... I've been there myself. One time I was going through some 'mods' and with each 'improvement' I made, it sound better. Then I started removing the mods, and it sounded better each time I removed what I'd done before. I was back to where I began. Looked myself in the mirror and laughed.
 
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Sold Dynaco in the late 60's. My boss told me that folks were saying their old tube Dyna stuff sounded better than the PAT-4/ST-120 I wanted to order (and did). That was clearly crazy. Took me probably 5 years to hear enough tired old traded in ST-70's to realize he was tellin me the truth. Back then ST-70/PAS3x's were close to a dime a dozen ( I owned only a couple of the PAS pre-amps -but four or five ST-70's in the late seventies-early 80's) -the simple trick was to retube them ( and I cry when I remember what I paid for Telefunken tubes back then). Original power specs on Dyna tube stuff introduced in the late 50's-early 60's was at 1kHz (not 20-20khz). I seem to remember the "corrected" power output of a stereo 70 was 28 WPC at rated distortion 20-20kHz ? These are old and let's face it-nobody was ever impressed by the quality of Dyna/Hafler input RCA's :) . Today -they are going to be a project rather than just a simple retube. Replaced a lot of Selenium rectifiers/5AR4 tubes with SS diodes - at some point realized that my cheap customers who didn't want to pay for this simple mod were getting better output tube life compared to the units I had "upgraded" to SS rectifiers/OTOH good 5AR4's are going for pretty fancy money last time I looked.
 

anmpr1

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Sold Dynaco in the late 60's. My boss told me that folks were saying their old tube Dyna stuff sounded better than the PAT-4/ST-120 I wanted to order (and did). That was clearly crazy. Took me probably 5 years to hear enough tired old traded in ST-70's to realize he was tellin me the truth.

I seem to remember the "corrected" power output of a stereo 70 was 28 WPC at rated distortion 20-20kHz ?

Today -they are going to be a project rather than just a simple retube.
Starting in the late '60s, and by the '70s, the big mass market was in acoustic suspension type speakers. Small package, cheap to manufacture, and fairly even sound from top to bottom. Many were 4 ohm, and not too sensitive. AR3a, and so forth. I suspect that an ST-70 would be a good match, not strictly for power, but because of soft clipping tubes. But I'm just guessing; the AR thing was not my thing, and they never got my dollars.

The FTC schedule was not helpful for tube amps. The rule certainly did away with ridiculous claims (IHF power, peak power, music power...), so that was good, I guess.

By the early '70s transistors amps became more reliable. Everyone wanted a transistor amp. If you had a lot of dollars and wanted power you could get a Crown DC-300, at 150 per channel. Shortly thereafter, Bob Carver started selling his 700 watt (350/ch) monster amplifier, mainly to cope with the then power requirements of low sensitivity speakers. The AR LST could take 1000 watts for short bursts. Bob wrote an article about it, in Audio magazine, which still makes interesting reading. Prior to that time, 60 solid state watts per channel was considered a lot. In another few years Japanese were selling receivers with more watts than the Crown, and almost as much as the Phase Linear.

I advise everyone to build (refurb) a Dynakit. Not because of sound. But because it's fun. And we need more fun and enjoyment in the world. :)
 

SIY

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Anyone who thinks an ST-70 soft clips has never examined the clipping behavior of that amp. Worse yet, after it clips, it shows blocking distortion.

Agreed about the fun part, though, and that's why I use tubes (though I design my circuits to avoid blocking).
 
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I think a lot of the Dynaco tube reputation is because of the era they were being sold in. PAS-3X preamps were frankly pretty noisy thru their phono inputs (the only critical input at the time) -but the mk-III and St70 were useable with quieter SS preamps. Since fairly quickly SS amps were out powering tube amps it came down to Dyna compared to what ? In my store in 1968 - I can't think of a single SS power amp I'd want to own today -but a Dyna tube amp pops up at a thrift or estate sale - I'll snap it up ! Nelson Pass's first watt concept may have something to do with this . Assuming fresh tubes -those first few watts out of the tube amps of that era were a very pleasant listening experience at the volume levels most speakers were played at most of the time. And hi-fi tube amps had been made long enough that they really outshined that eras mostly unreliable (often grossly unreliable) initial transistor offerings.The choice was pretty much gradually increasing distortion as tubes aged or small fires and silence from almost all the initial SS offerings.
 
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anmpr1

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...hi-fi tube amps had been made long enough that they really outshined that eras mostly unreliable (often grossly unreliable) initial transistor offerings.The choice was pretty much gradually increasing distortion as tubes aged or small fires and silence from almost all the initial SS offerings.
The first (or one of the first) SS amps advertised was in early 1963--the Acoustech. Germanium transistor based with the following specs: 40 watts rms both channels driven into 8-16 ohms, 20-20kHz with less than 0.95% harmonic/IM distortion. It sold for $395.00, which is over 3 thousand of today's inflato dollars. So it wasn't cheap. Considering a Dynakit could be had for a fraction of the cost, the market spoke. I never heard much about the company. It was all before my time. Evidently Acoustech also made an electrostatic speaker, and some history search indicates they were bought by Koss. I remember when you could get a Dynakit mail order for next to nothing. The only thing that mattered was a soldering iron and your time. Who would have imagined that these things are still supported and modified by hobbyists?
 

SIY

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The ST-120 was not exactly known for its reliability, either...
 

sergeauckland

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The Leak Stereo 30 was also from the same era, launched I think in 1963. It used germanium transistors and offered a mighty 10 watts into 4 or 15 ohms (8 ohms power output wasn't specified.) Distortion was 0.1% at 1kHz, 8 watts into 15 ohms.

It had a tendency to thermal runaway, and although it was specced at 0.1% distortion, that was at 1kHz only, it did rather worse at LF and HF and crossover distortion was noticeable at low powers. It was quickly replaced by the Stereo 30+, which used silicon transistors and was a much better amplifier indeed, one still usable today into a benign load.

S
 

levimax

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Who would have imagined that these things are still supported and modified by hobbyists?
Authoritative data is a little sketchy but there are references to Dynaco ST-70 selling over 300,000 units which would make it the all time best selling amplifier in history.... some say it sold more than all other amps of the era combined.
 
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My favorite ST-70 trade in story was a guy who had a PAS-2 and ST-70 -we negotiated a trade in sight unseen (IIRR -$25 for the PAS-2 and 50 for the ST-70-if they worked of course). Didn't see the guy for months ?? Turned out he had forgotten he had built a wall in his basement some years ago and the ST-70 was behind that wall -working away- but from his end the trade in was a little more complicated than he had thought. The only company I can remember having reasonably reliable initial SS units was MacIntosh -the autoformer let their output transistors lead a relatively charmed life. OTOH -didn't enjoy listening thru them much.
 

Sal1950

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I advise everyone to build (refurb) a Dynakit. Not because of sound. But because it's fun. And we need more fun and enjoyment in the world.
Yep, who can call themselves a real audiophile that has never owned a Dyna ST 70?
I had one for many years, all original AFAIK except for tubes, that alternated off a display shelf, to powering a system in my workshop just for fun. Also on a couple occasions it stood in for the VTL Compacts driving my La Scala's when I needed to replace a noisy input tube or something, a place where it sounded way over it's niche with the Klipsch's 104 db sensitivity.
I got called back to the glamor of tubes in the modern era mainly due to their esthetic appeal, that glow of the firebottles and their beauty on the audio shrine. LOL But while investigating the market at the time, while looking to replace my Phase Linear 700B I didn't want to give up too much of SS's tech superiority. I ended up going for David Manley's early entry's into the US market with his Ultralinear Only (no IMO market magnet triode switches). It was this John Atkinson bench report, along with the many positive subjective reviews that pushed me over the edge for VTL. The best of both worlds was my take on things, besides the fact that my two monster HSU subwoofers would be SS powered. To their credit I got around 20 years of service from those amps with only a few tube replacements and one repair of a poor wave soldering joint in the feedback circuit of one amp. Only gave up those amps along with the rest of the system in my move to retirement digs..

"On the test bench, you could have been forgiven at first for thinking that this VTL was a solid-state design, the small-signal frequency response (1V into 4 ohms) extending from 2.2Hz to 77kHz (–3dB). Noise levels, too, were impressively low, measuring –90dB, unweighted, with the input shorted, this improving by just over 6dB when an A-weighting network was switched in circuit ahead of the meter."
Read more at https://www.stereophile.com/content...er-amplifier-measurements#9GE4cJFeTqkzGVU6.99

Back to the OP, ST 70s are HiFi royalty, everyone should have owned at least one, wish I had the room to have kept mine.
EBay Sales pictures of my girl below. :( LOL
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